The idea of developing working tools that can work automatically is old. Such tools are for instance robots for vacuum cleaning or to cut grass. Despite the fact that this is an old idea, such tools have not reached the consumer until recently. One example is the robotic vacuum cleaner Trilobite™ and the automatic lawnmower Automower™. Both of these treat (cut or clean) a surface by moving in relation to it within the area that should be treated.
In order to keep the robot within the area that should be treated search systems have been developed. These systems consist in at least one electrical cable together with a sensing system in the robot that detects signals transmitted by the cable. The cable/-s is for instance arranged in order to define a borderline which the robot is not allowed to pass so that it leaves the area it should treat. A robotic vacuum cleaner normally only uses such cables in door openings and at stairs since it normally works inside a room surrounded by walls. If the vacuum cleaner should operate in a very large room, cables dividing the room into different areas could be used. Robotic lawnmowers on the other hand do not work in areas defined by walls. Therefore, electrical cables defining the area or areas inside which the robot should operate are needed.
Such cables and also certain cables used with robotic vacuum cleaners can consist in permanent magnets or electrical cables through which an electric current may be transmitted.
The sensing system normally consist in at least one receiver unit that detects magnetic signals, a control unit connected to the receiver which process the received signals and a motor unit connected to the control unit which controls the movements of the robot. The system detects the variation in field intensity (the field being generated by the current or the permanent magnet) when the robot approaches the cable. The control unit processes the information and decides, depending on the function activated, to operate the movement of the robot by operating the motor unit. For instance, the system can prevent the robot from moving out from the area which it should treat or make it move along a cable.
A scarcity with present search systems for automatic robots is that they use an uninterrupted current (such as one or several sinus waves) in order to generate the magnetic signals. The magnetic fields that the current generates propagate in the whole area or parts of the area within which the robot intends to operate. If any other magnetic field, for instance generated by another electrical cable arranged near the area, propagates in the same area the sensing system will detect this field. Such disturbing field can confuse the sensing system and thereby cause operating problems for the robot. Especially signals from other similar search systems, such as the next-door neighbours' system, could cause such operating problems since both systems may operate within the same frequency band. The search systems will interfere with each other since the fields are added. Another problem with present search systems is that the costs for sinus wave systems has increased since these often requires trimming of the frequency defining components. The present invention has been developed in order to solve the above-mentioned problems.